Bacteriology: syphilis serology and prozone: practice question

A Microbiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Microbiology · Bacteriology: syphilis serology and prozone

Sample item

A 25-year-old woman has a diffuse maculopapular rash involving the palms and soles with condylomata lata. VDRL on undiluted serum is reported non-reactive. Clinical suspicion of secondary syphilis remains very high. What is the most appropriate next step?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Repeat the VDRL on serially diluted serum to exclude a prozone phenomenon

In secondary syphilis very high antibody titres can saturate antigen and prevent the flocculation reaction in neat serum, giving a false-negative result called the prozone phenomenon. Diluting the serum restores the reaction. A treponemal test such as TPHA or FTA-ABS should also be done. T. pallidum cannot be grown on routine laboratory media.

Option by option

  • A. Repeat the VDRL on serially diluted serum to exclude a prozone phenomenon (correct)

    Correct. Dilution removes antibody excess and unmasks the reactive test.

  • B. Conclude that syphilis is excluded because the VDRL is negative

    A single non-reactive undiluted result cannot exclude syphilis in a patient with classic features of secondary disease.

  • C. Repeat the VDRL after 6 months

    Delaying 6 months would leave an infectious patient untreated and does not address the false negative.

  • D. Culture the skin lesion on blood agar

    Treponema pallidum cannot be grown on blood agar or any routine laboratory medium, so culture cannot confirm the diagnosis.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Microbiology question

What is the answer to this Microbiology question?

The best answer is A, Repeat the VDRL on serially diluted serum to exclude a prozone phenomenon. In secondary syphilis very high antibody titres can saturate antigen and prevent the flocculation reaction in neat serum, giving a false-negative result called the prozone phenomenon.

Why is "Conclude that syphilis is excluded because the VDRL is negative" not the answer?

A single non-reactive undiluted result cannot exclude syphilis in a patient with classic features of secondary disease.

Why is "Repeat the VDRL after 6 months" not the answer?

Delaying 6 months would leave an infectious patient untreated and does not address the false negative.

Why is "Culture the skin lesion on blood agar" not the answer?

Treponema pallidum cannot be grown on blood agar or any routine laboratory medium, so culture cannot confirm the diagnosis.

What concept does this question test?

It tests Bacteriology: syphilis serology and prozone in Microbiology. Diluting the serum restores the reaction. A treponemal test such as TPHA or FTA-ABS should also be done. T. pallidum cannot be grown on routine laboratory media.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Bacteriology: syphilis serology and prozone?

Make a one-line identity card for each organism: shape and stain, defining test, toxin, disease and drug. Then use the flashcards and the other Microbiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.